Treatment of subterranean formations

ABSTRACT

A method of treating a subterranean formation by contacting the formation with the following: (a) ammonium sulfamate; (b) an oxidizing agent selected from a perchlorate or a nitrite or combinations thereof; and (c) an acid.

TECHNICAL FIELD

This invention relates to treatment of subterranean formations, forexample to fracture formations and/or stimulate hydrocarbon, for exampleoil and/or gas, production.

BACKGROUND

Oil and gas reserves trapped within low permeability reservoirs such asshale and tight-gas formations are difficult and expensive to recoverusing traditional methods. To maximise the production from suchformations, an extensive and complex fracture network must be created.One of the most commonly employed methods is hydraulic fracturing.Whilst hydraulic fracturing does create fractures, the extent andcomplexity of the fracture patterns may be insufficient to maximise oilrecovery and furthermore using high volumes of fracturing fluids iscostly and can damage formations. Thus, there is a need for additionalmethods to further increase the productivity of an oil producing well,for example a hydraulically fractured well. A suitable method of asecondary fracturing operation generates new (micro)fractures, extendsexisting (micro)fractures or opens up naturally occurring fractures.This secondary fracturing method increases the complexity of thefracture network and stimulated reservoir volume (the total volume ofreservoir rock that has been fractured).

A wide range of fracturing methods and formulations has been proposed.However, known methods may be costly and/or use corrosive chemicals.Additionally, some known methods involve introduction and/or productionof carbon dioxide within a formation. Production of large quantities ofcarbon dioxide in a formation may be undesirable because it may absorbinto oil and/or disadvantageously produce carbonic acid which may beundesirable.

There is an on-going need to develop improved fracturing methods andchemicals.

It is known to generate heat and gas in downhole operations for use in asecondary fracturing operation. However, known methods produce a limitedamount of gas. The pressures experienced within the formation means thata large amount of gas needs to be generated to produce a pressuresufficient to overcome the confining pressure within the wellbore.

The present invention is based, in preferred embodiments, on treatment,for example fracturing, of subterranean formations by use of a mixtureof chemicals which are arranged to undergo an exothermic reaction and/orproduce large quantities of gas underground. The combination of heat andgas pressure can be used to treat, for example fracture, the formation.The combination of heat and gas may create new fractures, extendexisting fractures or create microfractures within a hydraulic fracture.In preferred embodiments, the mixture of chemicals generates large gasvolumes per mole of reactants in the mixture and releases non-toxicby-products. In addition, in preferred embodiments, a relatively lowlevel or suitably substantially no carbon dioxide is produced byreaction of chemicals used in the treatment.

SUMMARY

It is an object of the present invention to address problems associatedwith fracturing and/or stimulation of formations.

DETAILED DESCRIPTION

According to a first aspect of the invention, there is provided a methodof treating a subterranean formation, the method comprising contactingthe formation with the following:

(a) ammonium sulfamate; and

(b) an oxidizing agent selected from a perchlorate or a nitrite orcombinations thereof; and/or

(c) an acid (e.g. acid (AA) referred to herein).

In one embodiment, said method may comprise contact with the compoundsreferred to in (a) and (b); or in (a) and (c). Preferably, however, themethod comprises treatment of the subterranean formation with thecompounds referred to in (a), (b) and (c).

A preferred perchlorate is an alkali metal perchlorate with sodiumperchlorate being especially preferred.

Preferably, said oxidizing agent is selected from a perchlorate or anitrite; and, more preferably, said oxidizing agent comprises aperchlorate or a nitrite, but not both.

Preferably, as between a perchlorate and nitrite, a nitrite ispreferred. Said nitrite is preferably arranged to provide nitrite ionsin aqueous solution.

Said nitrite may be selected from alkali metal nitrites, alkaline earthmetal nitrites, ammonium nitrite or organic nitrites. Said nitrite ispreferably selected from lithium nitrite, sodium nitrite, potassiumnitrite, calcium nitrite, magnesium nitrite, ammonium nitrite andcombinations thereof. Said nitrite is preferably sodium nitrite.

Said ammonium sulfamate and said oxidizing agent, especially saidnitrite, are preferably contacted so that they react and nitrogen isgenerated in the formation. In one embodiment, no carbon dioxide may begenerated in the formation using the method.

A ratio (A) is defined as the number of moles of ammonium sulfamatedivided by the number of moles of said oxidizing agent contacted withthe formation. There is no minimum or maximum amount of nitrite requiredfor the invention and so ratio (A) may be any value greater than 0.Ratio (A) may be from 0.05 to 2.0, for example 0.1 to 0.8; andpreferably ratio (A) is 0.2 to 0.6.

The method preferably comprises contacting the formation with saidammonium sulfamate, said oxidizing agent (e.g. said nitrite) and an acid(AA). Said acid (AA) may be a monoprotic acid, a salt of a monoproticacid, a polyprotic acid, a partial salt of a polyprotic acid, or a saltof a polyprotic acid.

Said acid (AA) may suitably be any water soluble Brønsted acid.

Said acid (AA) may be a mineral acid, an inorganic acid or an organicacid.

Acid (AA) may be selected from: HCl, HF, formic acid, lactic acid,phosphoric acid, sulfamic acid, ammonium bisulfate, acetic acid, oxalicacid, nitric acid, carbonic acid, nitrous acid, sulphuric acid, citricacid, propionic acid, lactic acid, trihaloacetic acids and acidprecursors that provide the acid under wellbore conditions such asanhydrides, salts of trihaloacetic acid for example ammoniumtrichloroacetate, or esters like triacetin, methyl acetate, or dimers,oligomers or polymers of hydroxy acids such as lactic acid. Acid (AA)may be selected from: HCl, sulfamic acid and ammonium bisulfate ormixtures thereof. Acid (AA) preferably includes one or more nitrogenatoms.

Said acid (AA) is preferably arranged to react, for example with othermaterials contacted with the formation to produce a gas, whereinsuitably the gas produced includes nitrogen atoms originating in theacid (AA). Thus, the method is preferably a method of treating asubterranean formation to generate gas within the formation. The gas maybe arranged to fracture the formation in a region adjacent to, or withinan area where said gas is produced.

Reference herein to a gas is intended to cover products which aregaseous at standard temperature and pressure (STP) (0° C. and 1 atm).

Preferably, said gas produced by reaction of said acid (AA) as aforesaidis a nitrogen atom containing gas (e.g. N₂).

Thus, by use of acid (AA) as aforesaid, the acid can be treated toproduce gas which can supplement gas produced by reaction of saidammonium sulfamate and said oxidizing agent, for example nitrite.

Preferably, said acid (AA) includes a nitrogen-atom.

Preferably, said acid (AA) includes a moiety

for example a moiety

or a moiety

Said acid (AA) may include a NH₂ moiety such as found in sulfamic acidor in ammonium bisulfate (e.g. wherein the NH₂ moiety is part of a NH₄ ₊ion).

Said acid (AA) is preferably selected from sulfamic acid and ammoniumbisulfate. A mixture of acids, for example the aforesaid acids may beselected.

A ratio (B) defined as the number of moles of ammonium sulfamate dividedby the total number of moles of acid (contacted with the formationand/or reacted with ammonium sulfamate and oxidizing agent in theformation) may be greater than 0 and may be 10 or less. Ratio (B) may bein the range 0.1 to 2.5, suitably in the range 0.2 to 1.5, preferably inthe range 0.4 to 1.1.

The total number of moles of acid may comprise the sum of the number ofmoles of acid (AA) and any other acid contacted with the formationand/or reacted with sulfamate and oxidizing agent in the formation.

In a preferred embodiment, acid used in the treatment may itself donateatoms (other than hydrogen atoms) to a gas generated in the method. Aratio (C) defined as the number of moles of ammonium sulfamate dividedby the sum of the number of moles of one or more acids which arearranged to react, for example with other materials contacted with theformation, to donate atoms (other than hydrogen atoms) to a gas (e.g.nitrogen) produced in the method may be greater than 0 and may be 10 orless. Ratio (C) may be in the range 0.1 to 2.5, suitably in the range0.2 to 1.5, preferably in the range 0.4 to 1.1.

In order to donate atoms (other than hydrogen atoms) to a gas produced,the acid may include a nitrogen atom. A ratio (D) defined as the numberof moles of ammonium sulfamate divided by the sum of the number of molesof one or more acids (contacted with the formation and/or reacted withsulfamate and nitrite in the formation) which include a nitrogen atom,for example a NH₂ moiety as described, may be greater than 0 and may be10 or less. Ratio (D) may be in the range 0.1 to 2.5, suitably in therange 0.2 to 1.5, preferably in the range 0.4 to 1.1.

Preferred acids which donate atoms to a gas as described may be sulfamicacid and ammonium bisulfate. A ratio (E) defined as the number of molesof ammonium sulfamate divided by the sum of the number of moles ofsulfamic acid and ammonium bisulfate (contacted with the formationand/or reacted with sulfamate and nitrite in the formation) may begreater than 0 and may be 10 or less. Ratio (E) may be in the range 0.1to 2.5, suitably in the range 0.2 to 1.5, preferably in the range 0.4 to1.1.

A ratio (H) defined as the number of moles of oxidizing agent divided bythe total number of moles of acid (contacted with the formation and/orreacted with said ammonium sulfamate and oxidizing agent in theformation) may be in the range 0.5-10, preferably 0.6-5.0, morepreferably 0.75-4.0 and most preferably from 0.9-3.5.

A ratio (I) defined as the number of moles of oxidizing agent divided bythe sum of the number of moles of one or more acids which are arrangedto react, for example with other materials contacted with the formation,to produce a gas (e.g. nitrogen) as described may be in the range0.5-10, preferably 0.6-5, more preferably 0.75-4.0 and, especially,0.9-3.5.

Thus, preferably, the acid does not simply catalyse another reaction,but rather is directly involved in gas generation by donating atomsother than hydrogen (e.g. by donation of nitrogen atoms) to the gasproduced.

Said ammonium sulfamate may be provided as a slurry, an emulsion or asolution. Said ammonium sulfamate may be provided in water and themethod may comprise selecting an aqueous solution of ammonium sulfamate.The solution may be of any suitable concentration up to a saturatedsolution. Said ammonium sulfamate may or may not be encapsulated, forexample with an encapsulant arranged to delay reaction with the nitriteand/or acid (AA) on contact therewith. Said ammonium sulfamate ispreferably not encapsulated

Said oxidizing agent, for example said nitrite may be provided as aslurry, an emulsion or a solution. Said nitrite may be provided in waterand the method may comprise selecting an aqueous solution of saidnitrite. The solution may be of any suitable concentration up to asaturated solution. Said nitrite may or may not be encapsulated, forexample with an encapsulant arranged to delay reaction with thesulfamate and/or acid (AA) on contact therewith. Said nitrite ispreferably not encapsulated.

When the method includes use of acid (AA) as described, acid (AA) may beprovided in water for example as a solution or slurry in water. Saidacid (AA) may or may not be encapsulated. Said acid (AA) is preferablynot encapsulated, for example with an encapsulant arranged to delayreaction with the ammonium sulfamate.

In addition to the production of gas as described, said method may alsoproduce heat to facilitate treatment of the formation.

Said method of treating said subterranean formation may be used in anysubterranean formation that may benefit from the gas or heat rapidlygenerated by the reaction, for example to facilitate hydrocarbonproduction. The method may comprise treatment of said subterraneanformation in a drilling operation, a stimulation operation, a hydraulicstimulation operation, a sand control operation, a completion operation,a scale inhibiting operation, a water-blocking operation, a claystabilizer operation, a foam fracturing operation, a frac-packingoperation, a gravel packing operation, a wellbore strengtheningoperation, a sag control operation, an acidising operation, an alkalinetreatment operation, deposit removing operation, a ‘Huff and Puff’operation, in a process for inhibiting ‘frac hits’, a wellbore damageremoval operation, clean-up of a perforation, reduction of thehydrostatic pressure of the well, free stuck coiled tubing and/or pipe,a reservoir re-pressurisation operation, a depletion control operation,for far-field hydraulic fracture diversion, to reduce proppant settling,to reduce sand settling, an operation for increasing fracturecomplexity, or a fracturing operation.

Said method of treating a formation may be a ‘Huff and Puff’ operation.

‘Huff and Puff’ is a process that re-pressurises the near well area ofthe reservoir and reducing the viscosity of the oil in the surroundingformation. The reduction in oil viscosity can be achieved bypressurising the reservoir with a gas or fluid, comprising carbondioxide which dissolves into the oil and reduces its viscosity. Thepressurisation of the reservoir may be achieved by using any of thegas-generating reactions according to the invention. A typical ‘Huff andPuff’ operation would comprise a first step (i) of placing the gasgenerating chemicals within the wellbore and reacting them until thedesired pressure is reached and a second ‘shut-in’ step (ii) wherein thewell is sealed. Said shut-in step may be a full day or overnight. Oncethe well is opened production can resume.

Said method of treating a subterranean formation may be a process forinhibiting ‘frac hits’.

A ‘frac hit’ occurs when wells have been drilled in close proximity andfractures formed in the more recently drilled well grow into and throughthe production area of the older well and in some cases cause damages tothe older well. Fractures preferentially propagate through theweaknesses within the formation and so increasing the pressure in andabout the old well can divert and/or deflect the new fractures away fromthe older wells. The pressurisation of the older well can be achieved bycontacting the ammonium sulfamate, oxidising agent, especially saidnitrite, and acid (AA) within the older wellbore. This may be carriedout as a one off treatment or the ammonium sulfamate, oxidising agent,especially said nitrite, and acid (AA), may be continuously injected tomaintain a desired pressure.

Said method may comprise treatment of said subterranean formation, forexample to fracture the formation or increase the complexity of afracture network and/or stimulate hydrocarbon, for example oil and/orgas, production. By stimulate hydrocarbon production we mean, providinga method that improves the flow of hydrocarbons from the formation intothe production well. More preferably, said method comprises treatment ofsaid subterranean formation to fracture the formation or increase thecomplexity of a fracture network to facilitate hydrocarbon, for exampleoil and/or gas, production. For example, said method may extend anexisting fracture, create new fractures or create microfracturesextending out from a hydraulic fracture.

Preferably, said method is used in: a stimulation operation, a hydraulicstimulation operation, a ‘Huff and Puff’ operation, in a process forinhibiting ‘frac hits’, a wellbore damage removal operation, clean-up ofa perforation, reduction of the hydrostatic pressure of the well,freeing stuck coiled tubing and/or pipe, a re-pressurisation operation,a depletion control operation, for far-field hydraulic fracturediversion, to reduce proppant settling, to reduce sand settling, anoperation for increasing fracture complexity, or a fracturing operation.

Said method of treating a formation may comprise a wellbore damageremoval operation.

Said method of treating a formation may be to free stuck coiled tubingand/or pipe.

Said method of treating a formation may comprise cleaning equipment, forexample drilling equipment such as coil tubing underground. Gas producedmay be arranged to clean equipment by the gas pressure blowing off oiland/or other solid/liquid contaminants from the equipment.

Said method of treating a formation may comprise a reservoirre-pressurisation operation.

Said method of treating a formation may comprise far-field hydraulicfracture diversion.

Said method of treating a formation may comprise reducing proppantsettling.

Said method of treating a formation may comprise a stimulationoperation.

In one embodiment, ammonium sulfamate described in (a) and an oxidizingagent selected from a perchlorate and a nitrite as described in (b) maybe injected into different wellbores such that the reactants diffusethrough the formation until they contact each other and react with eachother within the formation. In this case, preferably, the wellbores areadjacent to each other. The pressure and concentrations of the reactantsmay suitably be selected to control where within the formation thereaction substantially occurs. This method of placing the reactantsdownhole may be used for a reservoir re-pressurisation operation, adepletion control operation, a damage removal operation or an operationfor the far-field diversion of hydraulic fractures.

The subterranean formation may comprise a source rock comprisinghydrocarbons (e.g., oil or natural gas) and may include shale,sandstone, or limestone or mixtures thereof. Said subterranean formationmay be subsea.

Said method of said first aspect is preferably a method of treating saidformation to stimulate the formation, for example to facilitateproduction of hydrocarbons, for example oil or gas from the formation.The method may comprise treating the formation to create or enhance afracture in the formation. The method preferably comprises treatment ofa formation which has already been fractured, wherein the method isarranged to enhance an existing fracture network and/or stimulatefurther hydrocarbon production from an existing formation.

The method may include introducing proppant and/or microproppant intothe formation to enter fractures formed in the method. Proppant and/ormicroproppant may be included in a formulation introduced to theformation after the formation has been treated with said ammoniumsulfamate, oxidizing agent and optional other reagents as described.

The method may also include introducing the proppant and/ormicroproppant in one or more of the formulations used in said method, soas to prop any fractures or microfractures formed as a result of themethod.

Said method may comprise introducing said ammonium sulfamate, forexample in aqueous solution, into the formation. Said ammonium sulfamatemay be directed towards a region of said formation it is desired totreat, for example fracture and/or stimulate. Said method may involveintroducing said ammonium sulfamate via an injection well. Coil-tubing(or the like) may be used to direct the ammonium sulfamate towards saidregion.

Said method may comprise introducing said oxidizing agent, preferablysaid nitrite, for example in aqueous solution, into the formation. Saidoxidizing agent, preferably said nitrite, may be directed towards aregion of said formation it is desired to treat, for example fractureand/or stimulate. Said method may involve introducing said oxidizingagent, preferably said nitrite, into the subterranean formation, forexample via a suitable well, such as an injection well. Said method mayinvolve introducing said oxidizing agent, preferably said nitrite, intothe production well. Coil-tubing (or the like) may be used to direct theoxidizing agent, preferably said nitrite, towards said region.

Said method may comprise introducing said acid (AA), for example inaqueous solution, into the formation. Said acid (AA) may be directedtowards a region of said formation it is desired to treat, for examplefracture and/or stimulate. Said method may involve introducing said acid(AA), via an injection well. Coil-tubing (or the like) may be used todirect the acid (AA) towards said region.

In some embodiments of the method, said ammonium sulfamate and saidoxidizing agent, preferably said nitrite, are preferably not contactedwith one another above ground. They are preferably contactedunderground, during passage towards, or preferably adjacent to or withinthe region of said formation it is desired to treat.

In other embodiments, it may be desirable to contact ammonium sulfamateand oxidizing agent, preferably nitrite, above ground and suitablytaking a step to eliminate or minimise reaction between the reagents.Such a step may comprise controlling the pH of a formulation of ammoniumsulfamate and oxidizing agent, preferably nitrite. Such control mayinvolve inclusion of an alkali in the formulation of ammonium sulfamateand oxidizing agent, preferably nitrite, to increase and maintain the pHwithin a range where the ammonium sulfamate and oxidizing agent,preferably nitrite do not react. For example, the pH may be greater than7, for example at least 8.

In the method, acid (AA) is preferably not contacted with said ammoniumsulfamate and oxidizing agent, preferably said nitrite, above ground. Itis preferably contacted with said ammonium sulfamate and/or nitriteunderground, preferably within and/or adjacent to the region of saidformation it is desired to treat.

In the method, for example in fracturing of a formation by production ofgas within the formation, the sum of the wt % of a formulation (F1)(e.g. an aqueous formulation) comprising said ammonium sulfamate, aformulation (F2) (e.g. an aqueous formulation) comprising said oxidizingagent, preferably said nitrite, a formulation (F3) (e.g. an aqueousformulation) comprising said acid (AA) introduced into the formation isat least 80 wt %, preferably at least 90 wt %, more preferably at least98 wt % of the total weight of materials introduced into the formationas part of the fracturing of the formation by production of gas withinthe formation, as described. For the avoidance of doubt, theaforementioned sum of the wt % is not intended to include a formulation(eg an inert spacer) which may be introduced into the formation (and maycontact formulation (F1), (F2) and/or (F3)) but which does not includean active ingredient which is involved in production of gas in theformation as described herein.

In another embodiment of the method, for example in fracturing of aformation by production of gas within the formation, the sum of the wt %of a formulation (F3) (e.g. an aqueous formulation) comprising said acid(AA) and a formulation (F4) (e.g. an aqueous formulation) comprisingsaid ammonium sulfamate, said oxidizing agent, preferable said nitriteand an alkali, introduced into the formation is at least 80 wt %,preferably at least 90 wt %, more preferably at least 98 wt % of thetotal weight of materials introduced into the formation as part of themethod of treating of the formation by production of gas and/or heatwithin the formation, as described in the first aspect. For theavoidance of doubt, the aforementioned sum of the wt % is not intendedto include a formulation (eg an inert spacer) which may be introducedinto the formation (and may contact formulation (F3) and/or (F4)) butwhich does not include an active ingredient which is involved inproduction of gas in the formation as described herein.

The sum of the wt % of ammonium sulfamate and water in formulation (F1),when introduced into the formation, is suitably at least 80 wt %,preferably at least 90 wt %, more preferably at least 95 wt %.

The sum of the wt % of oxidizing agent, preferably said nitrite, andwater in formulation (F2), when introduced into the formation, issuitably at least 80 wt %, preferably at least 90 wt %, more preferablyat least 95 wt %.

The sum of the wt % of acid (AA) and water in formulation (F3), whenintroduced into the formation, is suitably at least 80 wt %, preferablyat least 90 wt %, more preferably at least 95 wt %.

The sum of the wt % of ammonium sulfamate, oxidising agent, preferablysaid nitrite, alkali and water in formulation (F4), when introduced intothe formation, is suitably at least 80 wt %, preferably at least 90 wt%, more preferably at least 95 wt %.

In another embodiment of the method, for example in fracturing of aformation by production of gas within the formation, a formulation (F5)may be provided, wherein said formulation is aqueous and comprises saidammonium sulfamate and said acid (AA). In the method, for example infracturing of a formation by production of gas within the formation, thesum of the wt % of formulation (F5) and a formulation (F2) (e.g. anaqueous formulation) comprising said oxidizing agent, preferably saidnitrite, is at least 80 wt %, preferably at least 90 wt %, morepreferably at least 98 wt % of the total weight of materials introducedinto the formation as part of the fracturing of the formation byproduction of gas within the formation, as described. For the avoidanceof doubt, the aforementioned sum of the wt % is not intended to includea formulation (eg an inert spacer) which may be introduced into theformation (and may contact formulation (F5) and/or (F2)) but which doesnot include an active ingredient which is involved in production of gasin the formation as described herein.

Any of formulations (F1), (F2), (F3), (F4) and (F5) may compriseadditional components commonly used in the treatment of subterraneanformations for example: biocides, breakers, co-solvents, corrosioninhibitors, cross-linking agents, fluid loss control additives, frictionreducers, iron control agents, oxygen scavengers, pH adjusting agents,proppants, microproppants, salts, scale inhibitors, surfactants, sulfidescavengers, viscosifying agents, clay stabilisers and the like.

Co-solvents may be used in any of formulations (F1), (F2), (F3), (F4)and (F5) to improve the solubility of the reagents in water and/or thethermodynamic stability of the solution. The co-solvents are preferablypolar solvents for example: alcohols, glycols, amides, esters, ketones,sulfoxides etc. Suitably, the co-solvents are methanol or formamide ormixtures thereof. Specific examples may be selected from methanol and/orformamide.

Any suitable method may be used to place reagents into a well and/ordeliver to a desired position in a formation. The well may be ahorizontal or vertical well. However, preferred methods keep selectedreagents isolated from each other until they reach the desired locationwithin the formation.

Coiled tubing may be used to place reagents downhole. In this case, theend of the tube is placed where gas generation is required. One solutionis pumped through the tubing and another solution along the casing. Forexample, Formulation (F3) may be pumped through the coil and formulation(F4) may be pumped along the casing.

Coiled tubing may be especially useful to place the reagents downholein: a fracturing operation, a perforation clean-up operation, a wellboredamage removal operation, an operation to reduce the hydrostaticpressure of a well, or to free stuck coiled tubing and/or pipe.

Spacers may be used to keep the reagents and/or compositions separateuntil they reach a desired position in the formation. In this technique,a fluid, preferably an inert fluid, would used to separate the twoformulations of reactive components. Typically with this technique, 5-10bbl of the inert fluid may be used. Examples of inert fluids suitablefor this technique include, but are not limited to, pure water and oil.

In one embodiment, the formulations (F1), (F2) and (F3) are introduced,in any order, with an inert spacer separating each of the formulations.Formulation (F3) may be used as a spacer to separate formulations (F1)and (F2),

In another embodiment formulations (F3) and (F4) may be introduced intothe formation with an inert spacer separating the two formulations.

Spacers may be used to place the formulations downhole in the followingoperations: reservoir re-operation, a depletion control operation, adamage removal operation, for far-field hydraulic fracture diversion, afracturing operation, to reduce sand or proppant settling.

The formulations may be provided as part of an emulsion, for examplewater-in-oil emulsions or double emulsions, for examplewater-in-oil-in-water. In a double emulsion, the inner water phase maybe a formulation e.g. (F3) and the outer water phase may be a differentformulation e.g. (F4).

In preferred embodiments described herein, the number of moles of gasgenerated per mole of reactants may be increased compared to prior artproposals.

The sum of the total weight in grams (g) of ammonium sulfamate,oxidizing agent, preferably said nitrite, and acid(s) (including orconsisting of acid (AA)) introduced into the formation is hereinreferred to as SUM-W. The sum of the total volume of gas (e.g. CO₂and/or N₂) in cm³ generated by reaction of ammonium sulfamate, oxidizingagent, preferably said nitrite, and said acid(s) is herein referred toas SUM-V. Preferably, in the method, the Reaction Efficiency is definedas the volume of gases produced divided by the weight of reactants (i.e.SUM-V divided by SUM-W). The Reaction Efficiency is suitably at least100 cm³/g, for example at least 160 cm³/g or at least 180 cm³/g,especially it is at least 190 cm³/g. It may be less than 300 cm³/g.

The Reaction Efficiency as described may suitably be calculated based onweights of the specified reagents selected and gas generated by reactionthereof in a reaction carried out under controlled conditions at thesurface, based on amounts of reagents which are to be introduced intothe formation, since measurements within the formation itself are notpractical. Values referred to are suitably measured at STP, unlessotherwise stated.

To minimise the quantity of one or more of the formulations leaking offinto the formation and to maximise the fracturing effect, it isdesirable that the gas is rapidly generated after the components havebeen contacted with each other. The gas generation may substantially becomplete within 10 minutes of all the components being contacted witheach other. Preferably, the gas generation is substantially completewithin 5 minutes of the components being contacted with each other.

The quantities of formulations introduced into the formation as part ofthe method may be suitably selected dependent on the features of theformation, for example the confining pressure, and the pressure requiredto achieve the desired effect of said method of treating said formation.Thus, it is anticipated that any quantity of the formulations may beused. However, preferably at least 1 bbl may be used, for example 10 to500 bbl, or from 100 to 350 bbl, preferably from 150 to 250 bbl.

The rate at which one of more of the fluids is injected may suitably beadjusted according to the method of treating the formation and themethod of delivering the components. For example, it may be injected ata rate sufficient to build up a pressure such as that it fractures theformation.

In some methods of treating a subterranean formation, it may bepreferable to generate pulses of higher and lower pressures within theformation. This effect may be achieved by repeatedly reacting a gasproducing formulation within the formation. Either mechanical, chemicalor combinations of mechanical and chemical methods may be used tocontrol the manner in which the formulations are contacted with theformation to produce a series of pressure pulses. Said pulses ofpressure may be created in treating a subterranean formation in a methodcomprising:

(i) introducing a first gas producing formulation into the formation sothe formulation produces a gas in the formation;

(ii) reducing the rate of gas production within the formation, so thepressure produced in this step is lower than in step (i) and may be 0;

(iii) introducing a second gas producing formulation into the formation,which formulation may be the same or different to the first gasproducing formulation, thereby to produce a pressure higher than in step(ii); and, optionally,

(iv) reducing the rate of introduction of said second gas producingformulation into the formation.

Steps (ii) and (iii) may be suitably repeated to produce furtherpressure pulses as required.

Steps (i) through to (iv) may be carried out continuously,intermittently or a mixture of continuously and intermittently.

In step (ii), the reduction of rate of gas production in the formationmay be achieved mechanically, for example by reducing or stopping theamount of one or more gas generating reagents being introduced into theformation.

Step (ii) may be achieved using chemical means. For example, in oneembodiment, step (ii) may be achieved by pumping an inert fluid e.g. aspacer in between the pumping of gas producing formulations. In anotherembodiment, step (ii) may be achieved by pumping an inert fluidconcurrently with the first gas producing formulation, so as to reducethe concentration of the gas producing formulation and the rate at whichthe gas is produced. Then, step (iii) may comprise stopping the pumpingof the inert fluid.

In some embodiments the gas generating reagents used in the gasproducing formulation used in step (i) may be non-stoichiometric. Inthis case step (ii) may occur when one of the reagents (herein reagent(P)) is consumed so gas generation stops, leaving an excess of theremaining reagents (herein reagents (Q)). Step (iii) may then compriseinjecting a formulation comprising an excess of reagent (P). Steps (i)to (iii) may be repeated with the injected formulations beingalternated. For example the method may comprise contacting the formationwith 20 bbl of a solution of ammonium compound and acid and 10 bbl of asolution of sodium nitrite in step (i). Step (ii) occurs when the 10 bblof sodium nitrite is consumed. Step (iii) may comprise injecting 10 bblor more of sodium nitrite to produce a second pressure pulse. If themethod is to be repeated, step (iii) may use a large excess of sodiumnitrite.

According to a second aspect of the invention, there is provided amixture for treating a subterranean formation, the mixture comprising:

(a) ammonium sulfamate; and

(b) an oxidizing agent selected from a perchlorate or a nitrite orcombinations thereof;

(c) an acid (e.g. acid (AA) referred to herein).

The mixture may be produced below ground, for example within asubterranean formation.

The mixture may include more than one acid, for example including acid(AA).

The sulfamate, oxidizing agent, acid, including acid (AA), may be asdescribed in the first aspect.

According to a third aspect of the invention, there is provided acollocation adjacent a subterranean formation and/or adjacent aninjection well of a subterranean formation, wherein said collocationcomprises (P), (Q) or (R) as described below:

(P) a formulation comprising ammonium sulfamate (e.g. formulation (F1)of the first aspect), which is preferably provided in a receptacle (e.g.a receptacle (A));

a formulation comprising an oxidizing agent (e.g. formulation (F2) ofthe first aspect), which is preferably provided in a receptacle (e.g. areceptacle (B)); and, optionally (but preferably)

a formulation comprising acid (AA) (e.g. formulation (F3) of the firstaspect), which is preferably provided in a receptacle (e.g. a receptacle(C));

(Q) a formulation comprising ammonium sulfamate and an oxidising agent,preferably a nitrite which is preferably provided in a receptacle; and,optionally (but preferably)

a formulation comprising acid (AA) which is preferably provided in areceptacle;

(R) a formulation (F5), wherein said formulation is aqueous andcomprises ammonium sulfamate and acid (AA), wherein said formulation ispreferably provided in a receptacle; and a formulation (F2) (e.g. anaqueous formulation) comprising oxidizing agent, preferably a nitritewhich is preferably provided in a receptacle.

The collocation suitably includes pipework for delivering theformulations into the subterranean formation. Receptacle (A) maycommunicate with a pipe (which may comprise coil tubing) arranged todeliver formulation (F1) into the formation. Receptacle (B) maycommunicate with a pipe (which may comprise coil tubing) arranged todeliver formulation (F2) into the formation. Receptacle (C) maycommunicate with a pipe (which may comprise coil tubing) arranged todeliver formulation (F3) into the formation.

According to a fourth aspect, there is provided the use of the followingfor gas generation in a subterranean formation:

(a) ammonium sulfamate;

(b) an oxidizing agent selected from a perchlorate or a nitrite orcombinations thereof; and, optionally,

(c) an acid (AA).

The number of moles of gas generated per mole of reactants may beincreased compared to prior art proposals.

The use may be as described in the first aspect.

Any feature of any aspect of any invention or embodiment describedherein may be combined with any aspect of any other invention orembodiment described herein mutatis mutandis.

WORKING EXAMPLES

A subterranean formation may be treated with reagents which are arrangedto react to produce a gas and/or heat within the formation. This maystimulate the formation by improving a fracture network within theformation, for example by creating new fractures, extending existingfractures, opening up naturally-occurring fractures or creatingmicrofractures. The examples which follow describe reagents which may beused in a treatment.

Example 1—General Procedure for Undertaking Reactions

An ammonium compound and a nitrite or perchlorate-containing compoundwere added to a round-bottom flask and dissolved in the minimum quantityof water. Suitable apparatus to measure gas released was arranged inposition and the solution heated with stirring to 75° C. Once thesolution had reached 75° C., a selected amount of acid also heated to75° C. was injected into the reaction vessel. The quantity of gasgenerated was recorded.

In Examples 2 to 4, use of ammonium sulfamate as the ammonium compoundwas compared with use of other ammonium compounds, namely ammoniumchloride and ammonium bicarbonate.

Examples 2 and 3—Comparison Between Ammonium Sulfamate and AmmoniumChloride Using Sulfamic Acid

In order to compare use of ammonium sulfamate with ammonium chloride,ammonium sulfamate was reacted with sulfamic acid (Example 2) and thegas volume determined. For comparison purposes, the same reaction andassessment was undertaken wherein the ammonium sulfamate was replacedwith ammonium chloride (Example 3). In each case, 30 mmol of sodiumnitrite was used as oxidizing agent.

Results are provided in the table below from which it will be noted thatsignificantly more gas is generated when ammonium sulfamate is usedcompared to use of the other ammonium compounds.

Gas generated on reaction with acid/ Example No. Ammonium Salt Acid cm32 ammonium sulfamate sulfamic 1320 3 ammonium chloride sulfamic  960(comparative)

Examples 4 to 6—Comparison Between Ammonium Sulfamate Other AmmoniumCompounds Using Hydrochloric Acid

The procedure described for Examples 2 and 3 was followed excepthydrochloric acid was used instead of sulfamic acid. Results areprovided in the table below.

Gas generated on reaction with acid/ Example No. Ammonium Salt Acid cm34 ammonium sulfamate hydrochloric 920 5 ammonium chloride hydrochloric330 (comparative) 6 ammonium bicarbonate hydrochloric 560 (comparative)

Again, it should be noted that significantly more gas was generated whenammonium sulfamate was used compared to use of other ammonium compounds.

It is found that the solubility of sulfamic acid is relatively low. Toaddress this, mixtures of sulfamic acid and ammonium bisulfate (which isextremely soluble) were used with the ammonium sulfamate as describedbelow for Examples 7 to 18.

Example 7

2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodiumnitrite (20 mmol) was added to a round-bottom flask. Suitable apparatusto measure gas release was arranged in position and the solution heatedto 75° C. Once the solution reached 75° C., 0.83 mL of a 12 M aqueoussolution of hydrochloric acid (10 mmol), heated to the same temperature,was injected into the reaction vessel. The quantity of gas generated wasrecorded.

Example 8

2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodiumnitrite (20 mmol) was added to a round-bottom flask. Suitable apparatusto measure gas release was arranged in position and the solution heatedto 75° C. Once the solution reached 75° C., 4.0 mL of an aqueoussolution containing sulfamic acid (7.5 mmol) and ammonium bisulfate (2.5mmol), heated to the same temperature, was injected into the reactionvessel. The quantity of gas generated was recorded.

Example 9

2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodiumnitrite (20 mmol) was added to a round-bottom flask. Suitable apparatusto measure gas release was arranged in position and the solution heatedto 75° C. Once the solution reached 75° C., 3.0 mL of an aqueoussolution containing sulfamic acid (5 mmol) and ammonium bisulfate (5mmol), heated to the same temperature, was injected into the reactionvessel. The quantity of gas generated was recorded.

Example 10

2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodiumnitrite (20 mmol) was added to a round-bottom flask. Suitable apparatusto measure gas release was arranged in position and the solution heatedto 75° C. Once the solution reached 75° C., 1.95 mL of an aqueoussolution containing sulfamic acid (2.5 mmol) and ammonium bisulfate (7.5mmol), heated to the same temperature, was injected into the reactionvessel. The quantity of gas generated was recorded.

Example 11

2.2 mL of an aqueous solution containing ammonium sulfamate (7 mmol),sulfamic acid (1 mmol) and ammonium bisulfate (9 mmol) were added to around-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Example 12

2.5 mL of an aqueous solution containing ammonium sulfamate (5 mmol),sulfamic acid (2.5 mmol) and ammonium bisulfate (7.5 mmol) were added toa round-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.5 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Example 13

2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol),sulfamic acid (1.5 mmol) and ammonium bisulfate (8.5 mmol) were added toa round-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Example 14

2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol),sulfamic acid (1.75 mmol) and ammonium bisulfate (7 mmol) were added toa round-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Example 15

2.2 mL of an aqueous solution containing ammonium sulfamate (5.5 mmol),sulfamic acid (2.63 mmol) and ammonium bisulfate (4.88 mmol) were addedto a round-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Example 16

2.2 mL of an aqueous solution containing ammonium sulfamate (6 mmol),sulfamic acid (2.24 mmol) and ammonium bisulfate (4.76 mmol) were addedto a round-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Example 17

2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol),sulfamic acid (2.50 mmol) and ammonium bisulfate (4.25 mmol) were addedto a round-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Example 18

2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol),sulfamic acid (2.50 mmol) and ammonium bisulfate (3.75 mmol) were addedto a round-bottom flask. Suitable apparatus to measure gas release wasarranged in position and the solution heated to 75° C. Once the solutionreached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite(20 mmol), heated to the same temperature, was injected into thereaction vessel. The quantity of gas generated was recorded.

Results for Examples 7 to 18 are provided in the table below.

mmol mmol mmol Gas Total Efficiency/ Example NH₄NH₂SO₃ NaNO₂ Acid acidgenerated/cm³ mass/g cm³ per g 7 5 20 Hydrochloric 10.0 460 2.95 156 8 520 Sulfamic/ammonium bisulfate (75:25) 10.0 740 2.97 249 9 5 20Sulfamic/ammonium bisulfate (50:50) 10.0 700 3.01 232 10 5 20Sulfamic/ammonium bisulfate (25:75) 10.0 660 3.06 216 11 7 20Sulfamic/ammonium bisulfate (10:90) 10.0 630 3.31 190 12 5 20Sulfamic/ammonium bisulfate (25:75) 10.0 660 3.06 216 13 6.25 20Sulfamic/ammonium bisulfate (15:85) 10.0 640 3.22 199 14 6.25 20Sulfamic/ammonium bisulfate (20:80) 8.8 660 3.07 215 15 5.5 20Sulfamic/ammonium bisulfate (35:65) 7.5 660 2.82 234 16 6 20Sulfamic/ammonium bisulfate (32:68) 7.0 665 2.83 235 17 6.25 20Sulfamic/ammonium bisulfate (37:63) 6.8 670 2.83 237 18 6.25 20Sulfamic/ammonium bisulfate (40:60) 6.3 650 2.77 235

Thus, ammonium sulfamate may advantageously be used to generaterelatively large quantities of gas and, suitably, without generation ofany carbon dioxide (or any carbon-containing gas).

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

The invention claimed is:
 1. A method of treating a subterraneanformation, the method comprising contacting the formation with thefollowing: (a) ammonium sulfamate; and (b) an oxidizing agent comprisinga nitrite; and (c) an acid (AA); wherein a ratio (B) defined as thenumber of moles of ammonium sulfamate divided by the total number ofmoles of acid contacted with the formation and/or reacted with saidammonium sulfamate and oxidizing agent in the formation is greater than0 and is 10 or less; wherein a ratio (A) defined as the number of molesof ammonium sulfamate divided by the number of moles of said oxidizingagent contacted with the formation is from 0.05 to 2.0; wherein saidammonium sulfamate and said nitrite are contacted with one anotherunderground; wherein the method comprises treating the formation tocreate or enhance a fracture in the formation; and wherein in the methodthe sum of the wt % of a formulation (F1) comprising said ammoniumsulfamate, a formulation (F2) comprising said oxidizing agent and aformulation (F3) comprising the acid (AA) contacted with the formationis at least 98 wt %, of the total weight of materials introduced intothe formation.
 2. The method according to claim 1, wherein saidoxidizing agent is sodium nitrite and wherein said ammonium sulfamateand said oxidizing agent are contacted so they react and nitrogen isgenerated in the formation.
 3. The method according to claim 1, whereinthe ratio (A) defined as the number of moles of ammonium sulfamatedivided by the number of moles of nitrite contacted with the formationand/or reacted in the formation is in the range 0.2 to 0.6.
 4. Themethod according to claim 1, wherein the method comprises contacting theformation with said ammonium sulfamate, said oxidizing agent and theacid (AA), wherein said acid (AA) is arranged to react to produce a gas,wherein the gas produced includes carbon and/or nitrogen atomsoriginating in the acid (AA).
 5. The method according to claim 4,wherein said acid (AA) is treated in the method to produce gas whichsupplements gas produced by reaction of said ammonium sulfamate and saidoxidizing agent.
 6. The method according to claim 5, wherein said acid(AA) includes a moiety,

and wherein said acid (AA) includes a nitrogen-atom.
 7. The methodaccording to claim 5, wherein said acid (AA) is selected from sulfamicacid and ammonium bisulfate; and mixtures of sulfamic acid and ammoniumbisulfate.
 8. The method according to claim 7, wherein the ratio (B) isin the range 0.4 to 1.1; and/or wherein a ratio (C) defined as thenumber of moles of ammonium sulfamate divided by a sum of a number ofmoles of one or more acids which are arranged to react with othermaterials contacted with the formation to produce a gas is in the range0.4 to 1.1; and/or wherein a ratio (E) defined as the number of moles ofammonium sulfamate divided by a sum of the number of moles of sulfamicacid and ammonium bisulfate contacted with the formation is in the range0.4 to 1.1.
 9. The method according to claim 1, wherein a ratio (D)defined as the number of moles of ammonium sulfamate divided by the sumof the number of moles of one or more acids which include a nitrogenatom contacted with the formation and/or reacted with ammonium sulfamateand/or said oxidizing agent in the formation is in the range 0.4 to 1.1;and/or wherein a ratio (H) defined as the number of moles of oxidizingagent divided by the total number of moles of acid contacted with theformation and/or reacted with said ammonium sulfamate is in the range0.9 to 3.5; and/or wherein a ratio (I) defined as the number of moles ofoxidizing agent divided by the sum of the number of moles of one or moreacids which are arranged to react to produce a gas are in the range 0.9to 3.5.
 10. The method according to claim 1, wherein said ammoniumsulfamate is provided as a slurry, an emulsion or a solution; whereinsaid oxidizing agent is provided in water; and wherein the methodincludes contacting the formation with an aqueous solution or slurry ofthe acid (AA).
 11. The method according to claim 1, wherein the methodcomprises contacting the formation with a formulation (F5) which isaqueous and comprises said ammonium sulfamate and the acid (AA) which isselected from sulfamic acid and ammonium bisulfate; and mixtures ofsulfamic acid and ammonium bisulfate.
 12. The method according to claim1, wherein: the sum of the total weight in grams (g) of ammoniumsulfamate, oxidizing agent and acid(s) introduced into the formation isherein referred to as SUM-W; the sum of the total volume in cm³ of gasgenerated by reaction of ammonium sulfamate, oxidizing agent and saidacid(s) is herein referred to as SUM-V; wherein, in the method, theReaction Efficiency is defined as SUM-V divided by SUM-W; wherein theReaction Efficiency is at least 100 cm³/g and is less than 300 cm³/g.13. The method according to claim 1, further comprising producing pulseswithin the formation by controlling contact and/or amounts of theammonium sulfamate, the oxidizing agent and/or the acid (AA).
 14. Amethod of treating a subterranean formation, the method comprisingcontacting the formation with the following: (a) ammonium sulfamate; and(b) an oxidizing agent comprising a nitrite; and (c) an acid (AA);wherein a ratio (B) defined as the number of moles of ammonium sulfamatedivided by the total number of moles of acid contacted with theformation and/or reacted with said ammonium sulfamate and oxidizingagent in the formation is greater than 0 and is 10 or less; wherein aratio (A) defined as the number of moles of ammonium sulfamate dividedby the number of moles of said oxidizing agent contacted with theformation is from 0.05 to 2.0; wherein said ammonium sulfamate and saidnitrite are not contacted with one another above ground; wherein themethod comprises treating the formation to create or enhance a fracturein the formation; and wherein in the method the sum of the wt % of aformulation (F1) comprising said ammonium sulfamate, a formulation (F2)comprising said oxidizing agent and a formulation (F3) comprising theacid (AA) contacted with the formation is at least 98 wt %, of the totalweight of materials introduced into the formation.
 15. The methodaccording to claim 14, wherein said oxidizing agent is sodium nitriteand said acid (AA) is selected from sulfamic acid and ammoniumbisulfate; and mixtures of sulfamic acid and ammonium bisulfate.
 16. Themethod according to claim 15, wherein said ratio (A) is in the range 0.2to 0.6; and said ratio (B) is in the range 0.4 to 1.1.
 17. A method oftreating a subterranean formation, the method comprising contacting theformation with the following: (a) ammonium sulfamate; and (b) anoxidizing agent comprising a nitrite; and (c) an acid (AA); wherein aratio (B) defined as the number of moles of ammonium sulfamate dividedby the total number of moles of acid contacted with the formation and/orreacted with said ammonium sulfamate and oxidizing agent in theformation is greater than 0 and is 10 or less; wherein a ratio (A)defined as the number of moles of ammonium sulfamate divided by thenumber of moles of said oxidizing agent contacted with the formation isfrom 0.05 to 2.0; wherein said ammonium sulfamate and said nitrite arecontacted with one another underground, and wherein: the sum of thetotal weight in grams (g) of ammonium sulfamate, oxidizing agent andacid(s) introduced into the formation is herein referred to as SUM-W;the sum of the total volume in cm³ of gas generated by reaction ofammonium sulfamate, oxidizing agent and said acid(s) is herein referredto as SUM-V; wherein, in the method, the Reaction Efficiency is definedas SUM-V divided by SUM-W; wherein the Reaction Efficiency is at least100 cm³/g and is less than 300 cm³/g.
 18. The method according to claim17, wherein the method comprises treating the formation to create orenhance a fracture in the formation.
 19. The method according to claim17, wherein said oxidizing agent is sodium nitrite and wherein saidammonium sulfamate and said oxidizing agent are contacted so they reactand nitrogen is generated in the formation.
 20. The method according toclaim 17, wherein the ratio (A) defined as the number of moles ofammonium sulfamate divided by the number of moles of nitrite contactedwith the formation and/or reacted in the formation is in the range 0.2to 0.6.
 21. The method according to claim 17, wherein the methodcomprises contacting the formation with said ammonium sulfamate, saidoxidizing agent and the acid (AA), wherein said acid (AA) is arranged toreact to produce a gas, wherein the gas produced includes carbon and/ornitrogen atoms originating in the acid (AA).
 22. The method according toclaim 21, wherein said acid (AA) is treated in the method to produce gaswhich supplements gas produced by reaction of said ammonium sulfamateand said oxidizing agent.
 23. The method according to claim 21, whereinsaid acid (AA) includes a moiety,

and wherein said acid (AA) includes a nitrogen-atom.
 24. The methodaccording to claim 21, wherein said acid (AA) is selected from sulfamicacid and ammonium bisulfate; and mixtures of sulfamic acid and ammoniumbisulfate.
 25. The method according to claim 24, wherein the ratio (B)is in the range 0.4 to 1.1; and/or wherein a ratio (C) defined as thenumber of moles of ammonium sulfamate divided by a sum of a number ofmoles of one or more acids which are arranged to react with othermaterials contacted with the formation to produce a gas is in the range0.4 to 1.1; and/or wherein a ratio (E) defined as the number of moles ofammonium sulfamate divided by a sum of the number of moles of sulfamicacid and ammonium bisulfate contacted with the formation is in the range0.4 to 1.1.
 26. The method according to claim 17, wherein a ratio (D)defined as the number of moles of ammonium sulfamate divided by the sumof the number of moles of one or more acids which include a nitrogenatom contacted with the formation and/or reacted with ammonium sulfamateand/or said oxidizing agent in the formation is in the range 0.4 to 1.1;and/or wherein a ratio (H) defined as the number of moles of oxidizingagent divided by the total number of moles of acid contacted with theformation and/or reacted with said ammonium sulfamate is in the range0.9 to 3.5; and/or wherein a ratio (I) defined as the number of moles ofoxidizing agent divided by the sum of the number of moles of one or moreacids which are arranged to react to produce a gas are in the range 0.9to 3.5.